U.S. Pat. No. 3,169,899 (Steuber) describes a process for flash-spinning plexifilaments, and laying those plexifilaments down on a moving receiver or collection surface to form a nonwoven fibrous web. One element of the Steuber equipment comprises an electrostatic device (i.e., a target plate and ion gun plexifilamentary strand to the moving receiver (e.g., a conveyor belt).
U.S. Pat. No. 3,689,608 (Hollberg et al.) discusses the need for improvements in the Steuber process to provide a high degree of dispersion and uniformity in sheets destined for certain uses, and notes that the requirements for aerodynamic stability of the fine fibril network and the requirements for uniform electrostatic charging are somewhat in opposition to each other. Hollberg et al. note further that there are two requirements for effective charging; a high density of ions of a single polarity and a high electric field intensity in the vicinity of the fibers. Hollberg et al. suggest operating at 75 to 100 percent of the maximum sustainable charge for the web.
U.S. Pat. Nos. 3,851,023 and 3,860,369 (both to Brethauer et al.) describe a further improvement in the Steuber process wherein an aerodynamic shield is added to provide an equal impedance flow path independent of the initial radial direction at which the web leaves the spinning orifice.
While the Hollberg et al. process and apparatus, as improved by Brethauer et al., operated satisfactorily at slower spinning and laydown speeds, it has been discovered that as the throughput per position and the moving collection surface (e.g., conveyor belt) speed increase, plexifilament dispersion and subsequent sheet uniformity deteriorate. Attraction and adhesion of the plexifilaments to the collection surface, collectively called pinning, become less satisfactory.
Moreover, as spinning throughput increases, gas jet velocity and volume between adjacent spin positions increase. The gas flow causes increased turbulence and greater plexifilament "dancing" which tend to overcome the electrostatic attraction of the charged plexifilaments to the grounded collection surface. In addition, increased belt speed creates aerodynamic forces which attempt to pull the web along the collection surface. If the aerodynamic forces from the spinning jet and the collection surface movement are strong enough, they will collapse the plexifilamentary web, create ropes or other defects, and form a sheet of inferior uniformity. Under some commercially practical conditions, the drag forces and the pinning forces may reach the same order of magnitude.
The straightforward solution to the problem of increasing the electrostatic charge on the plexifilaments is unsatisfactory, since the charge on the plexifilaments is already at 75-100% of the peak charge. Additional charge from the ion gun would result in secondary ionization and loss of charge on the web. Secondary ionization is characterized by a glow discharge at the trailing edge of the target plate between the target plate and the plexifilaments as the plexifilaments leave the target plate. Moreover, secondary ionization is characterized by arcs from the charged web to the collection surface. Such arcs can cause the plexifilamentary strands to collapse on one another in the form of ropes.
Clearly, what is needed is a process and an apparatus that overcome the deficiences inherent in the prior art. In particular, the process and apparatus should allow for enhancing the electrostatic field and pinning forces in the region above the collection surface. Other objects and advantages of the present invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.